Alkanolamine sweetening units are used for the removal of H.sub.2 S and CO.sub.2 from natural gases, enhanced oil recovery gases, refinery hydrodesulfurizer recycle gases, FCCU and Coker gas plant tail gases, LPG streams, and Claus sulfur recovery tail gases. The alkanolamines (AAmines) commonly used are ethanolamine, diethanolamine, methyl diethanolamine, diisopropanol amine, and triethanol amine. These compounds are weak bases in aqueous solution. When solutions of alkanolamines are contacted in packed, sieve plate, bubble cap, or valve tray columns with streams containing H.sub.2 S and CO.sub.2, the H.sub.2 S and CO.sub.2 dissolve into the alkanolamine solution. The following chemical reactions then take place: EQU H.sub.2 S+AAmine=AAmineH.sup.+ +HS.sup.- EQU H.sub.2 O+CO.sub.2 +AAmine=AAmineH.sup.+ HCO.sub.3.sup.-
General Eqn.: Acid Gases+Alkanolamine=Alkanolamine Salts of Acid Gases
The solution of water, unreacted alkanolamine, and alkanolamine salts is subjected to stream stripping to reverse the above reaction and remove H.sub.2 S and CO.sub.2 from the alkanolamine. The H.sub.2 S and CO.sub.2 removed from the alkanolamine can then be processed by Claus sulfur recovery, incineration, fertilizer manufacture, or other means.
H.sub.2 S and CO.sub.2 are not the only gases in the above referred to streams which form weak acids when dissolved in water. Other such acid gases, as they are commonly called, that may appear in gas streams treated with alkanolamine include SO.sub.2, COS, or HCN. These gases also undergo the same reactions as H.sub.2 S and CO.sub.2 to form alkanolamine salts. These salts, however, cannot be removed by steam stripping as are H.sub.2 S and CO.sub.2 salts. Thus, they remain and accumulate in the system.
Another problem is presented if oxygen gets into the alkanolamine system. Oxidation of acid gas conjugate base anions leads to the formation of other alkanolamine salts, most commonly salts of thiosulfate S.sub.2 O.sub.3.sup.= and sulfate SO.sub.4.sup.=. Alkanolamine salts are also formed with thiocyanate SCN.sup.- and chloride Cl.sup.-. These salts also cannot be regenerated by steam stripping.
In addition to the inorganic anions, the alkanolamine solution may also be contaminated with organic anions such as anions of formic acid (HCO.sub.2.sup.-) and acetic acid (CH.sub.3 CO.sub.2.sup.-) and the like.
Alkanolamine salts which cannot be heat regenerated, called heat-stable salts, reduce the effectiveness of alkanolamine treating. The alkanolamine is protonated and cannot react with H.sub.2 S and CO.sub.2, which dissolve into the solution. Also, accumulated alkanolamine salts are known to cause corrosion in carbon steel equipment which is normally used in amine systems. These salts are also known to cause foaming problems which further decreases treating capacity.
One procedure used to deprotonate the alkanolamine so it can react with H.sub.2 S and CO.sub.2 is to add an alkali metal hydroxide such as NaOH to the amine solution. The deprotonated alkanolamine can then be returned to H.sub.2 S and CO.sub.2 removal service. However, the sodium salts of the anions of the heat-stable salts are also heat stable, and are difficult to remove, and thus accumulate in the alkanolamine solution with attendant corrosion and foaming problems.
The alkanolamine solution containing alkali metal salts of anions which form heat-stable salts with such alkanolamine may be reactivated by contacting it with a cation exchange resin whereby alkali metal ions are removed from the solution. Thereafter, the cation exchange resin is regenerated with a dilute mineral acid.
The remaining alkanolamine solution still contains the anions such as thiocyanate which form heat stable salts with the alkanolamine. One process which is very effective in accomplishing the removal of thiocyanate and other anions is disclosed in copending patent application Ser. No. 07/693,837, filed May 10, 1991, which is hereby incorporated by reference.
In Ser. No. 07/693,837, an alkanolamine solution containing thiocyanate anions and other anions which form heat stable salts with such alkanolamine is reactivated by contacting the alkanolamine solution with a strong base anion exchange resin having a high affinity for thiocyanate anions as compared to the other anions, contacting the effluent solution from the aforesaid strong base anion exchange resin with a strong base anion exchange resin which has an affinity for the other anions, thereafter regenerating the first mentioned strong base anion exchange resin by contacting it with sulfuric acid to effect removal of thiocyanate anions followed by contacting said resin with alkali metal hydroxide to remove sulfate anions and thereafter regenerating the second mentioned strong base anion exchange resin by contacting it with an alkali metal hydroxide to remove the other anions.
It is also disclosed in Ser. No. 07/693,837 that when the alkanolamine solution contains heat-stable alkali metal salts of thiocyanate and other anions, the cation exchange resin may be regenerated by first contacting it with aqueous ammonia to preferentially displace alkanolamine from the resin without displacing alkali metal cations and thereafter the resin is contacted with a dilute mineral acid to displace the ammonia, metal cations, and any remaining alkanolamine. Regeneration of the two anion exchange resins is then carried out in the same manner as described above.
It is apparent that conjugate base anions of acids are present during various stages of the alkanolamine treating process and also during the procedures carried out to reclaim spent alkanolamine. It would be desirable to have a process for determining the concentration and type of anions present in the alkanolamine solution at various stages of the treating process to reduce costs associated with under circulation, high corrositivity and poor treating of amine streams. It would also be desirable to monitor and control alkanolamine reactivation processes in which anions are removed from the alkanolamine.